Infrared sensing devices and methods

ABSTRACT

An infrared sensor assembly for sensing infrared radiation from an object is disclosed. The infrared sensor assembly comprises a sensor array comprising a plurality of sensing elements, provided on or embedded in a substrate extending in a substrate plane. The sensor array comprises at least two infrared sensing elements, each infrared sensing element having a radiation responsive element providing a proportionate electrical signal in response to infrared radiation incident thereto and at least two blind sensing elements, at least one blind sensing element being interspersed among the at least two sensing elements, each blind sensing element being shielded from incident infrared radiation from the object and providing a proportionate electrical signal in response to parasitic thermal fluxes. The output of the sensor array is a function of the infrared sensing elements and of the blind sensing elements such that parasitic thermal fluxes are at least partly compensated for.

FIELD OF THE INVENTION

The present invention relates in general to infrared (IR) sensingdevices, and more specifically to a thermal infrared sensing device.

BACKGROUND OF THE INVENTION

IR sensors are used to measure temperatures and/or provide images ofremote objects in a scene by detecting the infrared radiation emittedfrom the target object impinging upon a sensing array. Likewise, IRsensors with a single pixel are used to measure the temperature of anobject that illuminates the whole field of view of that sensor.

Ideally the sensing array should output a uniform response when it viewsan infrared source that produces a uniform amount of radiation.

A sensing array typically comprises an array (grid) of pixels eachindividually responsive to infrared radiation. In for example amicrobolometer, each pixel consists of a thermally isolated “bridge” ofresistive material that is heated by incident radiation. The resistanceof the bridge varies with its temperature and this variation inresistance is used to generate an output related to the intensity ofincident radiation. Another example of thermal infrared sensing devicesare sensors based on thermopiles.

In practice infrared sensor arrays are subject to a large amount ofnon-uniformity between pixels i.e. when exposed to the same amount ofradiation each pixel produces a different response. The raw output fromsuch arrays is dominated by this effect and is not recognizable as animage. As this is the case, infrared cameras known in the art apply acorrection to the raw output of the array. A known correction is togenerate a table of individual correction factors to be applied to theoutputs of each pixel in the image.

Additional problems are encountered if the temperature of the sensingarray varies, for example as a result of local hot spots in the sensoror by external heat sources surrounding the sensor, as the appropriatecorrection factors also vary with temperature. When the temperature ofthe sensor array varies, the latter creates inevitable linear andnon-linear thermal gradients over the sensor.

A thermal gradient over the pixels results in a different pixeltemperature for each of the pixels. The thermal infrared sensor with itspixels is situated in a certain environment. That environment has atemperature and it could also be that the environment is closed by acertain boundary (e.g. a cap on top of the sensor or another object at acertain temperature). Because of the thermal heat transfer between theenvironment or cap or other object towards the pixels, the pixels couldbe heated up differently because of the fact that their pixeltemperatures are different or because of the fact that the thermalresistance from the environment to the pixels is different.

One way in which this problem is dealt with is to characterize the arrayperformance at one or more temperatures and then to use dedicatedsensors provided on the array, typically blind pixels, thermistors orsimilar, to measure the current array temperature. A temperaturedependent interpolation can then be carried out to estimate a suitableadjustment to the correction factors. However, due to the extremesensitivity of the correction factors to array temperature, it isdifficult to apply this technique sufficiently accurately to provide anaccurate temperature measurement.

There is still room for improvement or alternatives.

SUMMARY OF THE INVENTION

It is an object of embodiments of the present invention to provide asensor assembly suitable for sensing infrared radiation with goodthermal stability.

The above objective is accomplished by a method and device according tothe present invention.

The present invention relates to an infrared sensor assembly for sensinginfrared radiation from an object, the infrared sensor assemblycomprising a sensor array comprising a plurality of sensing elements,provided on or embedded in a substrate extending in a substrate plane,wherein the sensor array comprises at least two sensing elements, eachsensing element having a radiation responsive element providing aproportionate electrical signal in response to infrared radiationincident thereto (LIVE pixel) and at least two blinded sensing elements,at least one blind sensing element being interspersed among the at leasttwo sensing elements, each blinded sensing element being shielded fromincident infrared radiation from the object and providing aproportionate electrical signal in response to parasitic thermal fluxes,whereby the output of the sensor array is a function of the infraredsensing elements and of the blind sensing elements such that parasiticthermal fluxes are at least partly compensated for. The function is afunction of a sum of the signals of the plurality of the infraredsensing elements and a sum of the signals of the plurality of blindsensing elements such that parasitic thermal fluxes are at least partlycompensated for. In embodiments of the present invention at least twoblind sensing elements may be interspersed among the at least twosensing elements.

The function may be a subtractive function of a sum of the signals ofthe plurality of the infrared sensing elements and a sum of the signalsof the plurality of blind sensing elements such that at least linearand/or non-linear thermal gradients over the sensor array are cancelled.

The parasitic thermal fluxes can be due to parasitic infrared radiationincident on the blind sensing element or due to thermal conductancethrough the solid or gas phase.

It is an advantage of embodiments of the present invention that theoutput corresponding with a function of signals of the LIVE pixels andsignals of the BLIND pixels compensates for the parasitic thermaleffects induced by the environment, e.g. the cap temperature orenvironmental temperature which normally may induce thermal gradients.It thus is an advantage of embodiments of the present invention thataccurate detection of infrared radiation stemming from a scene can bemeasured.

It is an advantage of embodiments of the present invention that linearand non-linear gradients in a first and a second direction arecompensated.

It is an advantage of specific embodiments of the present invention thatnon-linear thermal gradients in the diagonal direction are compensated.

It is an advantage of embodiments of the present invention thatprocessing of the signals can be performed in an integrated or anexternal processor.

Where in embodiments of the present invention, reference is made to apixel array, reference is for example made to a matrix of pixels, butalso to a group of pixels forming a 2 dimensional collection of pixels.

Said subtractive function may correspond with a sum of the signals ofthe plurality of the infrared sensing elements minus a multiplication ofa proportionality factor and a sum of the signals of the plurality ofblind sensing elements.

Any of said sum of the signals of the plurality of the infrared sensingelements and/or said sum of the signals of the plurality of blindsensing elements may be a weighted sum wherein for each of the pixels aweighing factor is taken into account.

The center of the sensor array may be matched with the center of a capcovering the sensor array or with respect to an external environmentinducing non-linear thermal gradients. It is an advantage of embodimentsof the present invention that the matching of the pixel array with thecap or external environment inducing non-linear thermal gradients can beeasily obtained.

The blind sensing elements and the infrared sensing elements may beconfigured for each having a field of view substantially correspondingwith the field of view of the infrared sensor assembly.

The infrared sensor device may comprise a processor for subtractiveprocessing of the electrical signal output of the plurality of theinfrared sensor elements of the sensor array together with the output ofthe plurality of blind sensor elements of the sensor array, thuscancelling the signal distortion caused by thermal gradients of thesensor array. It is an advantage of embodiments of the present inventionthat the processing can be implemented in a hardware based as well as asoftware based processor.

Alternatively, the signal of the live and blind pixels may be subtractedfrom each other in the analog domain by connecting the pixels inanti-series so the generated voltage is subtracted. This is however onlypossible for thermal sensing elements which generate a voltage signal,such as thermopiles.

The infrared sensor assembly may further comprise means for sampling anddigitizing the output of the plurality of infrared sensing elements ofthe sensor array together with the plurality of blind infrared sensingelements of the sensor array.

Each row and/or each column of the sensor array may comprise at leastone blind sensing element.

The sensor array may comprise a same amount of infrared sensing elementsand blind infrared sensing elements.

Each row and/or each column of the sensor array may comprise a sameamount of infrared sensor elements and blind infrared sensor elements,and the sensor elements are arranged in a checkerboard arrangement.

The sensor array may be a 2×2 sensor array wherein the main diagonalelements of the sensor array only comprise blind sensing elements.

Each corner of the sensor array may comprise a blind sensing element andthe sensor array further may comprise at least one blind sensing elementinterspersed in between the at least two infrared sensing elements.

The sensor array may comprise a same amount of rows and columns.

The infrared sensor assembly may further comprise a scanning shiftregister for selecting a row of the sensor array to be read out.

The infrared sensing elements and blind infrared sensing elements eachmay have a same layout. It is an advantage of embodiments of the presentinvention that the infrared sensor elements and blind infrared sensorelements are perfectly matched.

Channels may be provided between the infrared sensor elements and blindinfrared sensor elements of the sensor array. It is an advantage ofembodiments of the present invention that the thermal resistance fromthe cap to the pixels is the same for all the pixels. It is an advantageof embodiments of the present invention that it is guaranteed that thepressure of the sealed environment is the same for all the pixels.

The present invention also relates to an infrared sensor assemblycomprising a sensor array for providing an image signal of a scene, thesensor array comprising a plurality of infrared sensing elements andblind infrared sensing elements, wherein the sensor array compriseschannels which are provided between the plurality of infrared sensorelements and blind infrared sensor elements.

The present invention further relates to a method for sensing aninfrared signal from an object using an array comprising a plurality ofinfrared sensing elements, the method comprising sensing a signal usingat least two infrared sensing elements comprising a radiation responseelement providing a proportionate electrical signal in response toinfrared radiation incident thereto, sensing a signal using at least twoblind sensing elements interspersed among the at least two sensingelements, each blind sensing element having a radiation responsiveelement being shielded from the object providing a proportionateelectrical signal in response to parasitic thermal fluxes, andprocessing the signals such that the output of the sensor array is afunction of the infrared sensing elements and of the blind sensingelements such that parasitic thermal fluxes are at least partlycompensated for. The function is a function of a sum of the signals ofthe plurality of the infrared sensing elements and a sum of the signalsof the plurality of blind sensing elements such that parasitic thermalfluxes are at least partly compensated for.

Said processing may comprise deriving the output as a subtractivefunction corresponding with a sum of the signals of the plurality of theinfrared sensing elements minus a multiplication of a proportionalityfactor and a sum of the signals of the plurality of blind sensingelements.

Said processing may comprise deriving the output as a subtractivefunction wherein any of said sum of the signals of the plurality of theinfrared sensing elements and/or said sum of the signals of theplurality of blind sensing elements is a weighted sum wherein for eachof the pixels a weighing factor is taken into account.

In one aspect, the present invention also relates to an infrared sensorassembly for sensing infrared radiation from an object, the infraredsensor assembly comprising a sensor array comprising a plurality ofsensing elements, provided on or embedded in a substrate extending in asubstrate plane, wherein the sensor array comprises at least two sensingelements, each sensing element having a radiation responsive elementproviding a proportionate electrical signal in response to infraredradiation incident thereto (LIVE pixel) and at least two blinded sensingelements, at least one blind sensing element being interspersed amongthe at least two infrared sensing elements, each blinded sensing elementbeing shielded from incident infrared radiation from the object andproviding a proportionate electrical signal in response to parasiticthermal fluxes, whereby the output of the sensor array is a function ofthe infrared sensing elements and of the blind sensing elements suchthat parasitic thermal fluxes are at least partly compensated for. Theblind sensing elements and the infrared sensing elements are configuredfor each having a field of view substantially corresponding with thefield of view of the infrared sensor assembly. The function may be afunction of the sum of the signals of the infrared sensing elements andthe sum of the signals of the blind sensing elements. Further optionalfeatures and advantages may be as described in parts of the otheraspects of the present invention.

Particular and preferred aspects of the invention are set out in theaccompanying independent and dependent claims. Features from thedependent claims may be combined with features of the independent claimsand with features of other dependent claims as appropriate and notmerely as explicitly set out in the claims.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a sensor array that can benefit fromembodiments of the present invention.

FIGS. 2(a)-2(c) and FIGS. 3(a)-3(c) illustrate different examples ofpossible configurations of interspersed sensing elements, according toembodiments of the present invention.

The drawings are only schematic and are non-limiting. In the drawings,the size of some of the elements may be exaggerated and not drawn onscale for illustrative purposes.

Any reference signs in the claims shall not be construed as limiting thescope.

In the different drawings, the same reference signs refer to the same oranalogous elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. The dimensions and the relative dimensions do notcorrespond to actual reductions to practice of the invention.

Furthermore, the terms first, second and the like in the description andin the claims, are used for distinguishing between similar elements andnot necessarily for describing a sequence, either temporally, spatially,in ranking or in any other manner. It is to be understood that the termsso used are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other sequences than described or illustrated herein.

Moreover, the terms top, under and the like in the description and theclaims are used for descriptive purposes and not necessarily fordescribing relative positions. It is to be understood that the terms soused are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other orientations than described or illustrated herein.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. It is thus tobe interpreted as specifying the presence of the stated features,integers, steps or components as referred to, but does not preclude thepresence or addition of one or more other features, integers, steps orcomponents, or groups thereof. Thus, the scope of the expression “adevice comprising means A and B” should not be limited to devicesconsisting only of components A and B. It means that with respect to thepresent invention, the only relevant components of the device are A andB.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly, it should be appreciated that in the description of exemplaryembodiments of the invention, various features of the invention aresometimes grouped together in a single embodiment, figure, ordescription thereof for the purpose of streamlining the disclosure andaiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the detailed description are hereby expressly incorporatedinto this detailed description, with each claim standing on its own as aseparate embodiment of this invention.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood bythose in the art. For example, in the following claims, any of theclaimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

A “sensing element” in the context of embodiments of the presentinvention is an element which receives a signal, processes it andprovides a measurable output. In particular embodiments, the signalrefers to electromagnetic radiation in the infrared region, and thesensing element may comprise an integrated circuit, a MEMS, a thermopileor similar measuring systems, which are capable to transform thereceived signal into an electric signal for example a voltage, which maybe then transferred to a readable output (for instance, a display).

Where in embodiments of the present invention reference is made to“cap”, reference is made to a cover that protects the sensing element.Some embodiments of the present invention comprise a semiconductor cap,for instance silicon or germanium—the invention not being limitedthereto. In exemplary embodiments of infrared sensor arrays according tothe present invention, the cap comprises a cavity which isolates thesensing element, which may assist in improving the signal-to-noiseratio.

Where in embodiments of the present invention reference is made to“infrared radiation”, reference is made to radiation in the wavelengthrange from 1000 nm to 25000 nm, advantageously in the wavelength rangefrom 5000 nm to 20000 nm

Where in embodiments of the present invention reference is made to“blind sensing element”, reference is made to blind reference detectorswhich may be shielded or otherwise not permitted to view the infraredsignal of the object to be measured. Such a sensing element also may bereferred to as a blind sensing element.

Where in embodiments of the present invention reference is made to“infrared sensing element”, reference is made to a sensing element thatmeasures the infrared radiation incident on the sensor assembly. It alsomay be referred to as an infrared sensing element.

Where in embodiments of the present invention reference is made to“channel” reference is made to a narrow body of air between the infraredsensor elements of the array. This may be provided by a gutter or agroove between the infrared sensor elements. There is no realrestriction on the lateral dimensions of the channel and these may befor example in the range 10 μm to 1 mm. In one particular example, theheight of the channel may for example range from 1 μm to 20 μm if thechannel is made in the interface layer between a cap and a CMOS wafer.When an etched groove is used, the height can be larger than 20 μm.

Where in embodiments of the present invention reference is made to“interspersed” reference is made to scattering among or between otherthings; this can be done in a random or distributed pattern manner, forexample by arranging blind infrared sensor elements among the infraredsensor elements at intervals. In embodiments where a 2×2 infrareddetector array is provided the blind infrared sensor elements may beprovided as the diagonal elements of the infrared detector array,resulting in an infrared detector array where the blind infrared sensorelements are interspersed between or among the infrared sensor elements,because part of the blind pixels are lying in between parts of theinfrared sensor elements. In a specific embodiment an infrared detectorarray may comprise alternating blind infrared sensor elements andinfrared sensor elements, defining a checkboard pattern, where the blindinfrared sensor elements are interspersed in a distributed patternmanner.

In a first aspect, the present invention relates to a thermal infraredsensor assembly, making reference to a device formed by a sensing arrayand a stacked cap, whereby the sensor assembly comprising a sensor arraycomprising at least two blind sensing elements, at least one blindsensing element or at least two blind sensing elements beinginterspersed in between or among the at least two infrared sensingelements. Such a device thus is especially suitable for sensingradiation in the thermal infrared region. By way of illustration,embodiments of the present invention not being limited thereto, anexemplary sensor assembly according to embodiments of the presentinvention is described with reference to FIG. 1 to FIG. 3, illustratingstandard and optional features.

Embodiments of the present invention provide a sensor array 500 whichcan be considered to be an infrared detector array. Referring nowparticularly to FIG. 1, an infrared radiation detector assemblycomprising an infrared radiation detector array 500, and morespecifically a 2×2 array, is shown in accordance with the presentinvention. The columns and rows of the array 500 are formed of aplurality of individual infrared sensing elements, like for exampleradiation detectors or “pixels” of the micro-cantilever type. Eachindividual detector 200, 201 includes an infrared radiation responsivemember for providing a proportionate electrical signal to processingcircuitry in response to infrared radiation incident to the detectorarray 500. The detectors in FIG. 1, illustrate blind infrared sensingelements or detectors 300, 301 wherein the sensing element is beingshielded from incident infrared radiation and providing a proportionateelectrical signal in response to parasitic thermal fluxes. The blindpixels 300, 301 are shown interspersed between the radiation detectors200, 201 as main diagonal elements of the infrared radiation detectorarray 500. Those skilled in the art recognize that positioning of theblind pixels is not limited to the specific arrangement shown, and thatalternative arrangements, some further examples being shown in FIGS. 2and 3, are within the scope of the invention. The sensing elements maybe separated from each other via channels, which physically separate thesensing elements.

The infrared sensor assembly 100 may have a conventional design, such asfor example illustrated in FIG. 1 illustrating an infrared sensor designhaving a cavity whereby the infrared sensor is covered by a cap 110.

In particular embodiments of the present invention, the cap 110 can bemade of a semiconductor material, such as for example germanium or, morepreferably, silicon crystal, or more generally any other material thatis transparent to the radiation of interest (infrared). Advantageously,such material also is substantially opaque (e.g. has a transmissioncoefficient of at most 0.2, for example at most 0.1) to any otherradiation, especially visible light, which is a common source of noisein infrared sensors. Embodiments of the present invention are notlimited in the way of fabrication of the pixel. In the example of FIG.1, the pixels are based on forming a cavity in a cap 110 and a cavity ina basic substrate being for example a CMOS, but embodiments of thepresent invention are not limited in the way of fabricating for examplethe cavity in the cap and the cavity in the CMOS. One example could beto create cavities by KOH etching, but other technologies exist too. Theetch depth in the CMOS and the etch depth in the cap can be the same orcan be different. Such a different etching depth can have a positive ornegative effect on the parasitic thermal fluxes, as can be easilychecked by the person skilled in the art.

Blind sensing elements can for example be created by providing forexample a reflective coating on a top surface of the cap 110substantially above the sensing element or for example a reflectivecoating on a bottom surface of the cap 110, although embodiments of thepresent invention are not limited thereto.

Once the sensing element receives the radiation, in particularembodiments of the present invention its energy is converted to anelectrical signal, for instance through difference of potential in athermopile, and the electrical signal is driven, for instance through anintegrated circuit for processing the signal.

FIGS. 2(a)-(c) illustrate embodiments of the present inventionillustrating how the at least two blind sensing elements areinterspersed when the sensor array 500 is a square array, with the samenumber of rows and columns. FIG. 2(a) illustrates a 2×2 sensor array 500according to an embodiment of the present invention employing the atleast two blind sensing elements 300, 301 in the main diagonal. Thelatter checkerboard concept of two active of live sensing elements andtwo blind sensing elements advantageously can compensate for linearand/or non-linear thermal gradients in a first and a second direction,e.g. in X- and Y-direction which may be orthogonal or may be notorthogonal with respect to each other.

FIG. 2(b) illustrates an alternative embodiment of a 4×4 sensor array500, having at least two blind sensing elements interspersed among theactive or live sensing element 200, 201. Finally, FIG. 2(c) illustratesinterspersing of the at least two blind sensing elements in acheckerboard pattern for a 4×4 sensor array, where a blind and livesensing element are alternatingly provided in the sensor array. Inembodiments where square sensor arrays are provided the at least twoinfrared sensing elements and at least two blind infrared sensingelements are preferably provided such that the elements are symmetricalwith respect to the diagonal D1, D2 of the array. In addition, said 4×4checkerboard pattern advantageously not only compensates for linearand/or non-linear thermal gradients in a first and a second direction,e.g. X- and Y-direction which may be orthogonal or may not be orthogonalwith respect to each other, and also compensate for non-linear thermalgradients in a diagonal direction of the pixel array.

FIGS. 3(a)-(c) illustrate embodiments of the present inventionillustrating where each corner of the sensor array comprises a blindsensing element and further at least one additional blind sensingelement is interspersed in between the sensing elements, whereby saidfurther blind sensing element enables one to know whether thedistribution of the parasitic signal on the whole sensor array is aconstant (due to no thermal gradients), or whether it's a non-lineardistribution, and one then also may obtain the magnitude of theparasitic signal on the central active pixels of the pixel array,thereby being able to compensate for the non-linear thermal parasiticsignal on all active pixels of the array.

Embodiments of the present invention are advantageous for infraredsensors and provide a solution for the problem that all the activepixels are placed at different locations and consequently these activepixels will also observe a different influence of the environmentalparasitic effects. Although one wants to be only sensitive to thetemperature of the object in the optical field of view of the sensor.The solution for compensating for these effects as employed inembodiments of the present invention is based on the fact thatcompensation can be done for thermal gradients, linear and non-linear,over the sensor using interspersed blind pixel.

Further, according to embodiments of the present invention, the outputof the sensor array is a function of the infrared sensing elements andof the blind sensing elements such that parasitic thermal fluxes are atleast partly compensated for.

The function may be a subtractive function of a sum of the signals ofthe plurality of the infrared sensing elements and a sum of the signalsof the plurality of blind sensing elements. By providing the output assuch a function, at least linear and/or non-linear thermal gradientsover the sensor array can be cancelled. Such a processing of the signalscan be for example performed in the digital field, after digitizing,where required, the signal outputs of the different sensing elements.Alternatively, the signal outputs of the different sensing elements canbe processed in the analog domain by connecting the pixels inanti-series so the generated voltage is subtracted. The latter ispossible where the signal outputs are voltage signals.

According to some embodiments, an infrared sensing element has a blindsensing element positioned on an isotherm occurring in the sensorassembly.

By way of illustration, embodiments of the present invention not beinglimited thereby, the following theoretical considerations can be made,illustrating features and advantages of embodiments of the presentinvention:

The heat flux from the environment or cap or other object to the sensing(LIVE) and reference (BLIND) pixels is a function of the temperature ofthe environment or cap or other object, the thermal resistance betweenthe environment or cap or other object and each of the LIVE and BLINDpixels and the pixel temperature of the LIVE and BLIND pixels. Supposethat the design of the sensor is made in such a way that the thermalresistance between the environment or cap or other object and the LIVEpixel is exactly the same as the thermal resistance between theenvironment or cap or object and the BLIND pixel, and suppose that theenvironment or cap or other object is at a uniform temperature for theheat transfer towards each of the pixels, then still an offset could becreated between the LIVE and BLIND pixels when their pixel temperaturesare different. And this pixel temperature difference is influenced bylocal hot spots in the sensor or by external heat sources surroundingthe sensor, which create inevitable linear and non-linear thermalgradients over the sensor and thus over the pixels. As a consequence,the subtracted signal of LIVE and BLIND pixels is sensitive to thetemperature of an object outside the Field of View or the environment orcap for which the sensor is not allowed to be responsive. Embodiments ofthe present invention are therefore based on taking into account afunction of outputs of the sensing elements and the reference elements,positioned interspersed between the sensing elements.

The solution provided in embodiments of the present invention are basedon the fact that superposition of linear thermal gradients over thesensing/reference elements can be assumed. Therefore, if one looks to athermal gradient in the horizontal X-direction (in-plane of the pixels)over the sensor, one can see that there exist isotherms on the sensorsurface in the Y-direction. So by placing a sensing elements andreference elements on this isotherm in the Y-direction, one cancompensate for thermal gradients in the X-direction. The latter can beobtained by using interspersed reference elements, positioned betweenthe sensing elements. The same reasoning can be applied for a thermalgradient in the horizontal Y-direction (in-plane of the pixels), wherebyadvantageously the sensing element and a reference element are placed onthe isotherm in the X-direction. Nevertheless, in some embodiments, thepixels can also be slightly misaligned compared to the isotherms. Suchmisalignments may occur e.g. by design and/or by processing. Positioningof the pixels on isotherms can be obtained using an interspersedconfiguration of measurement and reference pixels. Whereas theillustration above is discussed for perpendicular directions, theprinciple counts more generally when selecting isotherms in a first andsecond direction, which may or may not be orthogonal to each other andby applying a superposition principle for both directions.

As discussed above, processing of the signals according to embodimentsof the present invention is performed by applying a function of thesignals of the infrared sensing elements and the reference sensingelements. Some particular examples, embodiments of the present inventionnot being limited thereto are a subtractive function on the sum of thesignals of the measurement (live) sensing elements and the sum of thesignals of the reference (blind) sensing elements. Such an example of asubtractive function is given byOutput=Sum(Live_i)−K*Sum(Blind_j); i: 1 . . . N; j: 1 . . . Mwith OUTPUT the resulting signal that is envisaged, i a counter for theN infrared sensing elements, j a counter for the M blind sensingelements and K a proportionality factor.

It is to be noticed that a subtractive function of the sum of thesignals can be a mere subtraction of the sum of the signals of theinfrared sensing elements and the sum of the signals of the blindsensing elements. In the above equation, the proportionality factor thenbecomes 1.

Nevertheless, a proportionality factor different from 1 (K< >1) also maybe introduced to take into account a general variation for the blindsensing elements.

In other embodiments, the different sensing elements may show specificdependencies and in such a case an individual weighing factor may beused for the signal of each individual sensing element. The mathematicalequation then becomes:Output=Sum(c _(i)*Live_i)−Sum(c _(j)*Blind_j); i: 1 . . . N; j: 1 . . .M.

According to another aspect of the invention, at least one blindinfrared sensing element and a scanning shift register provides an imagesignal of reduced fixed pattern noise and temperature stability. Thescanning shift register may be adapted to select one of the infraredsensing elements which is read out and used to compensate the imagesensing radiation detector.

The invention claimed is:
 1. An infrared sensor assembly for sensinginfrared radiation from an object, the infrared sensor assemblycomprising: a sensor array comprising a plurality of sensing elements,provided on or embedded in a substrate extending in a substrate plane,wherein the sensor array comprises: at least two infrared sensingelements, each infrared sensing element having a radiation responsiveelement providing a proportionate electrical signal in response toinfrared radiation incident thereto; at least two blind sensingelements, at least one of said two blind sensing elements beinginterspersed among the at least two sensing elements, each of said twoblind sensing elements being shielded from incident infrared radiationof the object and providing a proportionate electrical signal inresponse to parasitic thermal fluxes, wherein an output of the sensorarray is a subtractive function of a sum of the signals of the pluralityof infrared sensing elements and a sum of the signals of the pluralityof blind sensing elements such that at least linear and/or non-linearparasitic thermal fluxes are at least partly compensated for.
 2. Theinfrared sensor assembly according to claim 1, wherein said subtractivefunction corresponds with a sum of the signals of the plurality of theinfrared sensing elements minus a multiplication of a proportionalityfactor and a sum of the signals of the plurality of blind sensingelements and/or wherein said subtractive function is achieved by placingthe sensors in circuit according to an anti-series schematic.
 3. Theinfrared sensor assembly according to claim 1, wherein any of said sumof the signals of the plurality of the infrared sensing elements and/orsaid sum of the signals of the plurality of blind sensing elements is aweighted sum wherein for each of the pixels a weighing factor is takeninto account.
 4. The infrared sensor assembly according to claim 1,wherein each of the blind sensing elements and the infrared sensingelements have a full field of view that corresponds with the field ofview of the infrared sensor assembly.
 5. The infrared sensor assemblyaccording to claim 1, the infrared sensor assembly comprising a capcovering the sensor array, wherein the center of the sensor array ismatched with the center of the cap covering the sensor array or thecenter of the sensor array is matched to an external environment thatinduces non-linear thermal gradients.
 6. The infrared sensor assemblyaccording to claim 1, further comprising a processor for processing ofthe electrical signal output of the infrared sensing elements and of theblind sensing elements, thus cancelling the signal distortion caused bythermal gradients of the sensor array, and/or further comprising aprocessor programmed for subtractive processing of the electrical signaloutput of the plurality of the infrared sensor elements of the sensorarray together with the output of the plurality of blind sensor elementsof the sensor array, thus cancelling the signal distortion caused bythermal gradients of the sensor array.
 7. The infrared sensor assemblyaccording to claim 1, further comprising means for sampling anddigitizing the output of the plurality of infrared sensing elements ofthe sensor array together with the plurality of blind infrared sensingelements of the sensor array and/or wherein parasitic thermal fluxesinclude a convolution of thermal gradients over the sensor array andwherein for each infrared sensing elements a blind sensing element ispositioned on an isotherm in the pixel array, the isotherm beingcorresponding with one of the thermal gradients in the convolutedthermal gradients.
 8. The infrared sensor assembly according to claim 1,wherein each row and/or each column of the sensor array comprises atleast one blind sensing element.
 9. The infrared sensor assemblyaccording to claim 1, wherein the sensor array comprises a same amountof infrared sensing elements and blind infrared sensing elements and/orwherein each row and/or each column of the sensor array comprises a sameamount of infrared sensor elements and blind infrared sensor elements,and the sensor elements are arranged in a checkerboard arrangement. 10.The infrared sensor assembly according to claim 1, wherein the sensorarray is a 2×2 sensor array wherein the main diagonal elements of thesensor array only comprise blind sensing elements.
 11. The infraredsensor assembly according to claim 1, wherein each corner of the sensorarray comprises a blind sensing element and whereby the sensor arrayfurther comprises at least one blind sensing element interspersed inbetween the at least two infrared sensing elements and/or wherein thesensor array comprises a same amount of rows and columns.
 12. Theinfrared sensor assembly according to claim 1, further comprising ascanning shift register for selecting a row of the sensor array to beread out.
 13. The infrared sensor assembly according to claim 1, whereinthe infrared sensing elements and blind infrared sensing elements eachhave a same layout.
 14. The infrared sensor assembly according to claim1, wherein channels are provided between the infrared sensor elementsand blind infrared sensor elements of the sensor array.
 15. An infraredsensor assembly comprising a sensor array for providing an image signalof a scene, the sensor array comprising a plurality of infrared sensingelements and blind infrared sensing elements, wherein the sensor arraycomprises channels which are provided between the plurality of infraredsensor elements and blind infrared sensor elements.
 16. A method forsensing an infrared signal from an object using an array comprising aplurality of infrared sensing elements, the method comprising: sensing asignal using at least two infrared sensing elements comprising aradiation response element providing a proportionate electrical signalin response to infrared radiation incident thereto; sensing a signalusing at least two blind sensing elements interspersed among the atleast two sensing elements, each of said two blind sensing elementsbeing shielded from incident infrared signal from the object, providinga proportionate electrical signal in response to parasitic thermalfluxes; and processing the signals such that the output of the sensorarray is a subtractive function of a sum of signals of the plurality ofinfrared sensing elements and a sum of the signals of the plurality ofblind sensing elements such that at least linear an/or non-linearparasitic thermal fluxes are at least partly compensated for.
 17. Themethod according to claim 16, wherein said processing comprises derivingthe output as a subtractive function corresponding with a sum of thesignals of the plurality of the infrared sensing elements minus amultiplication of a proportionality factor and a sum of the signals ofthe plurality of blind sensing elements.
 18. The method according toclaim 17, wherein said processing comprises deriving the output as asubtractive function wherein any of said sum of the signals of theplurality of the infrared sensing elements and/or said sum of thesignals of the plurality of blind sensing elements is a weighted sumwherein for each of the pixels a weighing factor is taken into account.